X-ray tube comprising an electron source with microtips and magnetic guiding means

ABSTRACT

An X-ray tube including an electron source and a magnetic guide. The X-ray tube includes at least one electron source, at least one microtip, and an extraction grid, one zone of which emits electrons. Further provided are at least one anode, one zone of which emits X-rays under the impact of the electrons, and a magnetic guiding device for the electrons, capable of creating a magnetic field which is homogeneous at least between the zones. Such an X-ray tube may find application to X-ray absorption analysis or X-ray fluorescence analysis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an X-ray tube comprising a microtipelectron source.

The invention applies most especially to X-ray absorption analysisthrough thin objects or thin layers, for example for taking radiographicobservations of thin objects with a very good resolution, provided theX-rays source (which forms part of the tube and is the point from whichX-rays are emitted) is extremely well defined, i.e. has clear-cut edgesand/or controlled intensity over the whole of the zone of emission; thiszone of emission can be of small dimensions or conversely very extended.

The invention also makes it possible to X-ray liquids circulating inunderground piping of very small dimensions and small thickness.

It is further applicable to the medical field and in particular tomammography from a localized source of X-rays.

The invention also applies to X-ray fluorescence analysis.

It is true that low-energy X-rays have short trajectories. It isnevertheless possible to make a fluorescence analysis of light elements(Ca, Mg etc.) by means of “soft” X-rays generated in a tube according tothe invention, and with great spatial accuracy, provided the X-raysource is extremely well defined.

In the case where the source of electrons present in a tube according tothe invention is constituted of several sources of electrons separatedfrom one another, it is possible, by exciting these sources one afterthe other, to make a series of several images in order to observe asample from several angles.

The thickness or the shape of this sample may then be known with greateraccuracy than with a conventional X-ray tube.

2. Discussion of the Background

The principle of the generation of X-rays in a conventional X-ray tubeis well known.

It is based on the production of X-radiation when a sufficientlyenergetic electron bombards an atom of the tube's target.

In a conventional X-ray tube, a potential difference (of at least 50 kVfor high energy tubes) is applied between the thermo-ionic cathode(usually, a very hot tungsten filament) and the tube's anode.

The current extracted from the filament strikes the anode (on a surfacewhich is more or less well defined depending on the configurations andthe means of focussing with which the tube is equipped), which generatesthe X-rays at the points of impact.

The anode can be subject to high voltage and the filament to a potentialclose to earth, or the anode can be at earth potential and the filamentnegatively polarised.

Only the potential difference counts.

The choice of the potential reference is thus free.

In a case where the anode is at earth potential and the filamentnegatively polarized, the anode is more easily cooled (hydraulically) toevacuate the heat dissipated by the electrons penetrating into thetarget (anode) material since the potential of this target is 0V, i.e.is equal to the potential of the water evacuated by pipes.

An X-ray tube of this type has the structure of a diode.

More complex tubes may include, as well as the anode and the filament,an intermediate grid the role of which is explained below.

Since the filament is hot (and therefore capable of emitting electrons),the grid potential is sufficiently close to that of the filament, sothat the electron cloud emitted by the filament remains held in the zonebetween the filament and the grid.

The sudden increase in the potential of this grid makes it possible toextract the electron cloud from this zone, and to let it reach the anodethrough the grid.

This grid is thus used as an “electron gate valve”.

It must not be mistaken for the extraction grid included in microtipcathodes, which provides extraction of the electrons according to quitea different physical principle (the field effect).

In other known X-ray tubes, the electrons are provided by the fieldeffect by means of the use of pointed needles.

The configuration is then that of a diode (the electrical field is theresult of the potential difference which exists between the anode andthe needles).

However, because of the rapid wearing out of these needles, these othertubes were not as successful as expected.

In conventional X-ray tubes, a certain focussing of the electrons is ingeneral provided by a suitable configuration of the anode-filamentassembly.

The electrons leave a certain zone of the cathode and reach the anode ina zone whose surface is limited.

The configuration of the anode-cathode assembly is best defined bycalculating the trajectories of the electrons in the region situatedbetween the anode and the cathode, using the formulae of electronicoptics.

However, the shape of the filaments (cathodes) does not always make itpossible to have an impact of predetermined shape on the anode, andconsequently the X-ray source, whose extension corresponds to the impactzone of the electrons, suffers from this defect.

Electron guns for X-ray tubes are also known which allow increasedfocussing of the electron beams.

In this case, X spots of smaller or better defined size are generated.

If, for example, the electron beam of an electron microscope (having asubmicronic diameter) is used, and if this beam is directed at a target,the result is the equivalent of a circular-shaped microfocus X-ray tube.

Such an electron microscope used as an X-ray tube generally has anelectron gun equipped with magnetic and electrostatic lenses in order tofocus the electron beam on a small surface.

Microtips are also known for their use in flat screens or in certaininstruments such as pressure gauges.

Cathodes having a matrix structure and a large surface which usemicrotips are also known, as is their use inside flat screens aselectron sources for the production of visible light bycathodoluminescence.

It is also known from the American patent application of Cha-Mei Tang etal., serial number 201,963, of Feb. 25, 1994, that an X-ray tube couldinclude a microtip cathode and electrostatic focussing means which areincorporated in the cathode itself. Such a structure does not make itpossible to obtain an extended, well delimited emitter zone, having acontrolled intensity over the whole zone.

Furthermore, the structure of X-ray tubes with filaments does not makeit possible to define any specific shape of the X-rays source, i.e. thezone of the tube from which the X-rays are emitted, in an accurate andcontrollable fashion.

SUMMARY OF THE INVENTION

The aim of the present invention is to remedy these disadvantages.

Its object is an X-ray tube comprising:

at least one electrons source one zone of which, called the first zone,is intended to emit electrons,

at least one anode one zone of which, called the second zone, isintended to emit X-rays under the impact of these electrons, and

guiding means or focussing means (focussing being taken here in thebroad sense of “guidance”) on to this second zone of the electronsemitted by the first zone,

this X-ray tube being characterized in that the electrons source is anelectrons source with at least one microtip and with an extraction grid,and in that the guiding means of the electrons are magnetic guidingmeans capable of creating a magnetic field which is homogeneous (i.e.which has a direction and intensity which are substantially constant orslowly variable spatially) at least in the volume between the first andsecond zones, the vectorial characteristics (intensity, direction) ofthis field being such that the second zone is homothetic to the firstzone.

The invention makes it possible to obtain a X-radiation source (secondzone) having the shape, the distribution of intensity (number of Xphotons emitted per second per unit of surface) or the desireduniformity of emission by judicious selection of the magnetic field (forexample parallel to the mean direction of propagation of the electrons)and the shape of the emitter cathode (first zone).

In other words, the combination

on the one hand of a microtip source, whose geometry and distribution ofmicrotips in the source are adapted to the geometry and the distributionof the desired X-radiation and,

on the other hand of magnetic guiding means, whose intensity anddirection are adapted to the homothetic (identical or inferior orsuperior) reproduction of the emitter zone of the electrons bothspatially and in intensity,

makes it possible to obtain an X-ray tube whose intensity and geometryare perfectly defined.

In particular, the intensity obtained can be spatially variable orconstant.

The direction of the field corresponds to the straight line passingthrough

on the one hand the centre of the zone emitting the electrons, and

on the other hand the centre of the zone emitting X-rays.

It should be noted that, in order to have an identical reproduction onthe anode of the zone emitting the electrons, the intensity of themagnetic field must be greater than or equal to a threshold beyond whichthere always exists a beam of electrons whose envelopes of thetrajectories are parallel.

Since it uses a microtip of a plurality of microtips to emit theelectrons, the X-ray tube which is the object of the invention has inparticular the following advantages as compared with a conventionalX-ray tube using a filament which emits electrons:

There is no pollution of the anode by material which has evaporated froma hot cathode, therefore there is no longer any need to “hide” thefilament with respect to the anode; the cathode with microtips(s) can bepositioned facing this anode.

The construction of the tube is simpler.

The electron source gives off no heat and thus the anode cannot melt, atleast at low power.

The cathode can be pulsed (the length of the pulses can be well below 1μs and can even reach 100 ps), and this ability to pulse the cathode isaccompanied by extremely flexible electronics, which do not affect thehigh voltage circuits.

The tube can be connected to a battery.

The zone irradiated by the electrons can be so irradiated uniformly(which is not the case with a filament); the X-rays source is thusuniform (or of controlled uniformity) and the edges of a large emitterzone are clear-cut.

The number of connections (vacuum-tight lead-throughs) remains small bycomparison with a tube in which focussing is provided by supplementaryelectrodes.

In the X-ray tube which is the object of the invention, the electronsource can comprise a single microtip or a plurality of microtipsdepending on the desired geometry and intensity of the X-ray emitterzone.

According to another variant, the X-ray tube includes a plurality ofelectron sources, an X-ray emitter zone corresponding to each electronsource.

The tube, the object of the invention can comprise one anode or aplurality of anodes, each anode then being associated with at least onemicrotip.

The electron source can be pulsed so as to obtain X-ray pulses.

The X-ray tube, the object of the invention can further comprise anelectrically conductive grid which is positioned between the electronsource and each anode, this grid being polarized so as to prevent theions from reaching the electron source and to avoid the formation ofelectric arcs between this electron source and each anode.

The magnetic guiding means, of the tube, the object of the invention cancomprise one or more magnets or Helmholtz coils or both magnets andHelmholtz coils.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by reading thedescription of example embodiments given below, purely as examples andin no way exhaustive or limiting, and referring to the appended drawingsin which:

FIG. 1 is a diagrammatic view of a specific embodiment of the X-raytube, the object of the invention, wherein the electron source comprisesonly a single microtip,

FIG. 2 is a diagrammatic view of another specific embodiment wherein theelectron source comprises a number of microtips,

FIG. 3 is a diagrammatic view of another specific embodiment whereinthere are a plurality of anodes,

FIG. 4 is a diagrammatic view of another specific embodiment wherein theanode is formed on the window of the tube, and

FIG. 5 shows diagrammatically regulating means of the electron source ofan X-ray tube according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the invention, to guide the electron beam emitted by the microtipelectron source and to direct this beam to a determined place, amagnetic field is used, the intensity of which can go from a fewhundredths of a tesla to a few tenths of a tesla, for example, thismagnetic field being, in the case of an identical reproduction of theelectron emitter zone, parallel to the median trajectory of the electronbeam.

In the rest of the description, for the sake of simplicity, the case ofa parallel field is considered.

It is well understood that the insertion can use a divergent orconvergent field to reproduce the said electron source zone in anenlarged or a reduced way.

It is known that the trajectories of the electrons then wind around thedirection of the magnetic field with a radius, the value which isinversely proportional to the intensity of this magnetic field.

The average trajectories of the electrons are then substantiallyparallel and scarcely diverge at all.

The zone called “spot” in which the electron beam meets the anode isthen identical to the zone in the source which emits the electrons if itis assumed that the anode is placed perpendicularly to the electronbeam.

The shape of the emitter zone of the electron source (cathode) is thusreproduced on the anode and the X-ray source thus has strictly this sameshape.

The density of X-ray emission depends on the density of the incidentcurrent, which in turn depends on the density of the microtips on thecathode and on the current emitted by each microtip.

A more complex magnetic configuration could if appropriate producegreater concentration of the electron beam rather than simply preventingit from diverging.

In this case the “spot” formed on the anode can be even smaller.

In the examples described below the zone which emits the X-rays has ashape which is homothetic with that of the zone which emits theelectrons if no account is taken of the angle of incidence of theelectrons on the anode (when the latter is different from 90°). This canin any case be corrected by giving the electron emitter zone a shapesuch that when projected on to the anode the spot obtained has thedesired shape.

It should also be noted that the X-rays generated at the surface of theanode are emitted isotropically.

Some of them escape from the anode while others penetrate more deeplyinto it.

If this anode is thick, the only usable X photons are those emitted outof the anode.

In each of the examples diagrammatically shown in FIGS. 1 to 4, an X-raytube is provided with a window made of a material selected to be asnon-absorbent as possible with respect to X-rays so that they can passthrough this window and leave the tube, or as thin as possible to limitabsorption (a membrane of nanometric thickness made of Si₃N₄ or SiC canbe used).

This window also maintains the airtightness of the enclosure of eachX-ray tube, in which enclosure is created (by means not shown in FIGS. 1to 4) a pressure which is sufficiently low (for example of the order of10⁻⁸ hPa or less) so that the X-ray tube will operate correctly anddurably.

In one specific embodiment not shown the X-ray tube is itself undervacuum (for example in the case of an electron microscope) and thiswindow is then eliminated or it acts only as an optical filter or apollution filter and the X-rays produced are then propagated in vacuoand irradiate a sample also placed in vacuo.

FIG. 1 is a diagrammatic view of a first example of the X-ray tubeaccording to the invention.

The X-ray tube diagrammatically represented in this FIG. 1 comprises inan enclosure under vacuum 2, an electron source 4 comprising a singlemicrotip 6, made of an electron-emitting material and formed on anappropriate substrate 8, and an incorporated extraction grid 16, thesource being preferably made using the techniques of microelectronics.

In the enclosure 2 there is also a single metallic anode 10 placedopposite the microtip 6.

Means not illustrated are provided to bring this anode 10 to a highpositive voltage with respect to the microtip 6.

The X-ray tube in FIG. 1 also comprises Helmholtz coils 12 preferablyplaced outside the enclosure 2 (which is made of an anti-magneticmaterial) these coils being provided for creating a magnetic field Bwhich is substantially parallel to the axis Z of the microtip and whichis homogeneous within the volume between the microtip and the anode 10,this volume being limited by the dot-dash lines t visible in FIG. 1.

Instead of coils 12 it is possible to use one or more magnets to createthis magnetic field and this magnet (these magnets) can be placed insideor outside the enclosure 2.

The voltage applied between the anode and the microtip can be of theorder of +5 kV to +50 kV.

An electron beam is then emitted by the microtip 6 in the direction ofthe axis Z towards the anode 10, by means of the application of avoltage to the extraction grid 16.

The microtip 6 is capable of emitting a current of the order of 100 μA.

This electron beam is concentrated and guided towards the anode 10 bythe magnetic field B.

This magnetic field is of the order of a few tenths of a tesla.

Since a single microtip is being used, the electron emitting zone is ofthe order of 1 μm² or less. The size of the electronic spot on the anodeis also of the order of 1 μm² or even less with more intense magneticfields.

Thus X-rays are generated (referenced X in FIGS. 1 to 4) from amicro-focus F1 whose size is of the order of 1 μm².

As can be seen in FIG. 1, the enclosure 2 is closed by a berylliumwindow 14.

The X-rays leave the anode 10, pass through the window 14 which istransparent to X-rays and which also ensures the airtightness of theenclosure.

These X-rays are then available for the use desired.

The X-rays generated in the anode 10 which are propagated within theanode (rearwards) are not used.

It should be noted that the microtip source 4 must be located at asuitable distance from the anode 10 so that:

the returning positive ions (which are propagated in the direction ofdecreasing potentials) do not damage the source or cathode 4, and

this cathode does not form a screen or shade to the emitted X-rays.

Preferably, to prevent ions from returning, an intermediate grid 17,which has high transparency to the electrons emitted by the microtip 6,is positioned between the source 4 and the anode 10, near the source 4,in the path of the electron beam, a few millimeters from the source 4.

This grid 17 is for example made of a conductive material and pierced asto 90% to allow the electrons to pass.

Furthermore, this grid 17 is raised (by means not illustrated) to apotential higher than that of the extraction grid 16. It can be eithervery much lower than that of the anode, for example of the order of 200V to 500 V, or again, if the grid is extremely transparent to electrons,slightly greater than that of the anode to prevent the positive ionsproduced at the anode by the impact of the electrons from returning asfar as the cathode.

A second example of the X-ray tube according to the present invention isdiagrammatically represented in FIG. 2.

The X-ray tube in FIG. 2 is similar to that in FIG. 1, except that inthe case of FIG. 2 the electron source 4 comprises a number of microtips6 which are formed on the substrate 8 and whose axes are substantiallyparallel.

The anode 10 is once more positioned opposite these microtips.

The magnet or the Helmholtz coils 12 are again provided for creating themagnetic field B which is homogeneous in the volume between 16 thesource 4 and the anode 10, this volume being limited by the dot-dashlines t visible in FIG. 2.

This magnetic field is substantially parallel to the axes Z of themicrotips.

The magnetic field B guides the electrons emitted by these microtips sothat the average trajectory of the electrons is substantially parallelto this magnetic field B in the volume limited by the dot-dash lines t.

Preferably a grid 17 which is transparent to electrons is positionedbetween the anode 10 and the source 4, a few millimeters from thelatter, as is seen in FIG. 2.

Means not illustrated again make it possible to polarize the anode 10positively with respect to the microtips 6, for example at a voltage ofthe order of +10 kV, and to raise the grid 17 to a potential higher thanthat of the grids 16 but much lower than that of the anode 10, orslightly higher than the latter.

The substrate has for example an area of the order of 100 m² to 1 mm²and comprises, for example, 100 to 1,000 microtips distributed over azone with an area equal to 100 μm² and making it possible to obtain anelectronic current of the order of 1 mA to 10 mA.

If no account is taken of the space charge of the electron beam, themagnetic guidance makes it possible to obtain an electronic spot F2 onthe anode 10 having the same size as the zone occupied by the microtipsof the cathode 4 (taking no account of the inclination of the anode 10with respect to the electron beam).

This inclination of the anode in the X-ray tube in FIG. 2 (as indeed inthe case of the X-ray tube in FIG. 1) is provided for sending a largequantity of X-rays in the direction of the beryllium window 14.

It should be noted that in the case of FIGS. 1 and 2, the dimensions ofthe electronic spots and thus of the X-ray spots on the anode 10 aredirectly related with the size of the electron sources (single microtipor set of microtips).

It is therefore possible to make X-ray tubes according to the inventionin which the X-rays emitting zone has exactly the dimensions and shapedesired for the intended application, the distribution of intensity ofthe X-rays emitting zone being a function of the distribution of theemission intensity of the first zone.

The X-ray tube according to the invention which is diagrammaticallyrepresented in FIG. 3 differs from that in FIG. 1 in that in addition tothe anode 10, it comprises another anode 18 which is positioned besidethe anode 10, and a supplementary microtip 6 a positioned on thesubstrate 8, opposite this other anode 18.

In this example there are thus two electron emitting zones and two X-rayemitting zones.

Thus separate electron beams can be generated which are still guided bythe magnetic field B, this field being homogeneous in the volume betweenthe microtip sources and the two anodes (this volume being once morelimited by the two dot-dash lines t visible in FIG. 3).

These separate electron beams make it possible to generate separateX-ray beams.

The anodes 10 and 18 are similarly inclined with respect to the electronbeams, as can be seen in FIG. 3, so that each sends a large quantity ofX-rays towards the window 14.

On the other hand, if it were desired to separate the two X-ray beams,the anodes could be differently inclined.

Rather than associating a single microtip with each anode, it would bepossible to associate several microtips with it.

The zones F3 and F4 which emit X-rays, respectively situated on theanodes, are homothetic with the two zones which emit electrons(respectively with on microtip or a set of microtips).

The advantage of an X-ray tube of the type shown in FIG. 3 resides inthe fact that the two anodes can be made of different materials.

Thus X-rays of different wavelengths can be generated.

The “polychromic” X-ray tube thus obtained enables discriminatoryinterpretations of certain experiments to be made using X-rays.

It is possible for instance to arrange that the anode 10 emits X-raysthe wavelength of which does not enable particles 20 contained in asample 22 situated outside the X-ray tube, opposite the window 14, to beshown up, a detector 24 being place behind this sample 22 (which is thusbetween the window 14 and the detector); and also to arrange that theanode 18 emits X-rays the wavelength of which does enable theseparticles to be shown up.

By subtraction a better knowledge of the nature and localization of theparticles 20 contained in the sample 22 is thus obtained.

The tube according to the invention which is diagrammaticallyrepresented in FIG. 4, again comprises an enclosure 2 under vacuumclosed by a window 14 which is transparent to X-rays and is for examplemade of beryllium.

In this enclosure there is once more a microtip cathode 4 opposite whichis positioned a grid 17 which is transparent to the electrons emitted bythe microtips 6.

The X-ray tube in FIG. 4 also comprises an anode 10 at earth potentialand consisting for example of a layer of tungsten which is deposited onthe beryllium window.

Polarisation means 28 are provided to raise the microtips formed on anappropriate substrate 8 to a negative voltage with respect to theextraction grid 16 and means 29 are provided to raise the cathodeassembly to a high negative voltage with respect to that of the anode.

The anode 10 formed on the window 14 is positioned opposite the grid 16and the microtips 6, and this anode is substantially parallel to thesubstrate 8 and the grid 16.

The X-ray tube in FIG. 4 also comprises a magnet 30 located outside theenclosure 2 and is provided of creating a magnetic field B perpendicularto the anode, homogeneous within the volume between the source 4 and theanode 10 and provided for focussing the electrons emitted by themicrotips on to this anode.

When the anode 10 is hit by the electrons emitted by the microtips itemits X-rays which pass through the beryllium window 14.

A spatial X-ray detector 32 is positioned opposite the window 14,outside the enclosure 2 of the X-ray tube.

FIG. 4 also shows a sample screen 34 partially opaque to X-ray, providedwith an opening 36 and positioned between the window 14 and the spatialdetector 32, the X-rays thus traversing this opening 36 before reachingthe detector.

This example illustrates the concept of plane radiography with anextended source X: only the regions of slight absorption (symbolized bythe hole 36) allow passage to the X-rays detected by the two-dimensionaldetector 32.

The X-ray tube in FIG. 4 has an extended focus F5 (zone which emits theX-rays) defined by magnetic guidance, this focus having a uniformitywhich can be constant or controlled.

With a large enough microtip cathode this zone F5 which emits the X-rayscan have an area of tens of cm².

Such a zone F5, which is by no means selective, is neverthelessperfectly limited by means of the magnetic guidance of the electronbeams.

The zone F5 in FIG. 4, which emits the X-rays, has strictly the samedegree of extension as the electron emitting zone (set of microtips)although the microtip cathode 4 is separated from the anode 10 byseveral millimeters.

Any desired shape could be given to the microtip cathode of an X-raytube according to the invention, for example the shape of a “P”.

The X-rays emitting zone would than also have the shape of a “P”, whichis not feasible with a conventional X-ray tube using anelectrode-emitting filament or a thermoionic anode.

An X-ray tube according to the invention can be pulsed.

Generally speaking, the high voltage applied to the anode of this tubemay be pulsed, so that the electrons are alternately attracted thenrepelled by this anode, or the electron source may be pulsed so that theelectron beam is alternately emitted and then not emitted.

For instance, the anode may be raised to the high voltage (constant overtime) and pulse the microtip cathode to generate electron peak currentsof several mA, in the form of pulses reaching a duration of 100 ps orless, and separated by dead times of longer or shorter duration.

In the case of a pulsed tube, the electron beam is still guided by theaction of a magnetic field as has been seen from the examples in FIGS. 1to 4.

Such a pulsed tube can be applied to pulsed X-photography.

In the invention, it is of course possible to use a microtip cathodewith a matrix structure and to control successively the various rows ofthis microtip cathode, which also corresponds to a pulsed mode operationof the X-ray tube of this cathode with matrix structure.

In the present invention, it is possible to use as an anode a plate ofaluminium or magnesium or a thin layer of tungsten formed by evaporationon to a heat-conductive substrate (in order to be able to evacuate theheat). The material of the anode is selected from the periodic table ofthe elements depending on the application.

It should be noted that the window 14 which closes the vacuum enclosure2 is sufficiently thick to ensure vacuum-tightness but sufficiently thinnot to excessively absorb the X-rays emitted when the X-ray tube isoperating. For small windows it is possible to use membranes ofnanometric thickness.

This window may have a honeycombed structure providing both rigidity andvacuum-tightness and transmission of the X-rays thanks to the lowerthickness.

The thickness of this window depends on its diameter and may be of theorder of 100 μm or less in places and in the case of membranes it may bemeasured in hundreds of nanometers.

If desired, a getter-type element may be placed in this enclosure 2 tomaintain a very low pressure.

It is possible to associate with an X-ray tube according to theinvention a system of regulation of the electronic current emitted bythe microtip cathode, as is shown diagrammatically in FIG. 5.

This figure shows the microtip cathode 4, where a single microtip 6 isillustrated, resting on a grounded conductive layer 38.

This layer 38 in turn rests on a silicon substrate 40.

The pierced grid 16 opposite the microtip and electrically insulatedfrom the layer 38 by a layer 42 of SiO₂ can also be seen.

The anode 10 of the X-ray tube can also be seen as well as means 44enabling an appropriate variable positive voltage to be applied to thegrid 16 with respect to the microtip 6 and means 46 enabling anappropriate high voltage to be applied to the anode 10 with respect tothe microtip.

A resistance 48 of value r is mounted between the earth and the negativeterminal of the means 46 for applying the high voltage to the anode.

The regulation system consists of an operational amplifier 50 whichcontrols the means 44 for applying voltage depending on a referencevoltage R fixed by the users and on the voltage picture of the currentflowing in the resistance 48.

More exactly, the electrons entering the anode 10 correspond to acurrent of intensity i.

This comes from earth, passes through the resistance 48 and by thesupply (application means) 46.

At the terminals of the resistance there exists a voltage V equal tor.i.

This voltage V is passed to the operational amplifier 50 and this lattercompares this voltage V with the reference voltage R corresponding tothe current desired by the user.

This regulation system is known.

The examples of the invention which have been described by reference toFIGS. 1 to 4 use flat anodes.

However, using another type of anodes, for example cylindrical “rotatinganodes” would remain within the scope of the invention.

Journal of Microscopy, vol. 156, n^(o) 2, November 1989, p. 247 to 251describes an X-ray projection microscope comprising of a microtipelectron source and an anode which emits X-rays under the impact of theelectrons. Magnetic lens is positioned near the electron source. Anelectrostatic deflection system is included between the lens and theanode.

U.S. Pat. No. 4,979,199 A describes an X-ray tube comprising anelectron-emitting filament and an anode which emits X-rays under theimpact of the electrons. A magnetic coil creates a magnetic electronfocussing field in a zone between the anode and the cathode.

U.S. Pat. No. 4,012,656 describes an X-ray tube comprising afield-effect emission cathode.

U.S. Pat. No. 3,665,241 discloses the use of a microtip electron sourcein an X-ray tube.

U.S. Pat. No. 3,518,433 describes an X-ray tube comprising a fieldemission cathode and an adjacent control electrode.

WO 87/06055 describes an X-ray tube comprising a rotating photo-cathodeand a rotating anode which receives the electrons emitted by thephotocathode and emits X-rays.

U.S. Pat. No. 3,783,288 describes an X-ray tube with pulsed fieldemission, comprising a conical anode opposite which a cathode made ofspaced needles is positioned,

DE 895 481 describes cylindrical electromagnetic lens comprising a splitsupport, such that the density of the lines of force shall be at amaximum in one part of this coil.

EP 0 473 227 describes an X-ray tube comprising a cathode, anaccelerating anode, a magnetic lens system to focus the electronsleaving the accelerating anode and an anode constituting a target toproduce the X-rays by electronic bombardment.

U.S. Pat. No. 3,883,760 describes a field emission X-ray tube comprisinga cathode made of a graphite fabric. Each thread of the fabric comprisesfilaments of graphite which constitute electron emitters.

What is claimed is:
 1. An X-ray tube comprising: at least one electronsource one zone of which, called first zone, is intended to emitelectrons; at least one anode one zone of which, called second zone, isintended to emit X-rays under the impact of these electrons, and guidingmeans on to this second zone of the electrons emitted by the first zone,this X-ray tube being characterized in that the electron source is anelectron source with at least one microtip and with an extraction grid,and in that the guiding means are magnetic guiding means capable ofcreating a magnetic field which is homogeneous at least in the volumebetween the first and second zones, the vectorial characteristics ofthis field being such that the second zone is homothetic with the firstzone.
 2. An X-ray tube according to claim 1, wherein the electron sourcecomprises a single microtip.
 3. An X-ray tube according to claim 1,wherein the electron source comprises a plurality of microtips.
 4. AnX-ray tube according to claim 1, comprising a plurality of electronsources, a X-rays emitting zone corresponding to each electron source.5. An X-ray tube according to claim 1, comprising a single anode.
 6. AnX-ray tube according to claim 1, comprising a plurality of anodes, eachanode being associated with at least one microtip.
 7. An X-ray tubeaccording to claim 1, wherein the electron source is pulsed so as toobtain X-ray pulses.
 8. An X-ray tube according to claim 1, furthercomprising an electrically conductive grid positioned between theelectron source and each anode, this grid being polarized so as toprevent the ions from reaching the electron source and to prevent theformation of electric arcs between this electron source and each anode.9. An X-ray tube according to claim 1, wherein the magnetic guidingmeans comprise one or more magnets or Helmholtz coils or both magnetsand Helmholtz coils.